Currently, crystalline silicon has the largest market share in the photovoltaics (PV) industry, accounting for over 80% of the overall PV market share. And although moving to thinner crystalline silicon solar cells is long understood to be one of the most potent and effective strategy for PV cost reduction (because of the relatively high material cost of crystalline silicon wafers used in solar cells as a fraction of the total PV module cost), utilizing thinner crystalline is fraught with the problem of mechanical breakage caused by thin and often large substrate sizes. Other problems include inadequate light trapping in the thin structure because silicon is an indirect bandgap semiconductor material. Further, it is difficult balance the requirement of high mechanical yield and reduced wafer breakage rate with high manufacturing yields in PV factories in a cost effective manner.
On a standalone crystalline silicon solar cell without support, moving even slightly thinner than the current thickness range of 140 μm-250 μm starts to severely compromise mechanical yield during manufacturing. Thin film silicon is particularly mechanically fragile causing manufacturing and processing difficulties. Thus, solutions directed to process very thin solar cell structures may utilize a cell process during which the cell is fully supported by a host carrier throughout, or a cell process which utilizes a novel self-supporting, standalone, substrate with an accompanying structural innovation.
Although, in the past, there have been attempts in solar industry to use carriers such as glass for thin substrates, these carriers have suffered from serious limitations including low maximum processing temperatures (in the case of glass) which potentially compromises the solar cell efficiency. There have also been attempts to make small area thin cells which do not have serious breakage concerns; however, large cell areas are required for commercial viability.
Achieving high cell and module efficiency with a low fabrication cost is critical in solar cell development and manufacturing. Back junction/back contacted cell architecture is capable of very high efficiency—primarily because there is no metal shading on the front side and no emitter on the front which helps result in a high blue response, and also because of the potentially low metal resistance on the backside. It is known to those versed in the field that back contacted cell demands a very high minority carrier diffusion length to substrate thickness ratio (while a good criteria to have for any solar cell architecture including front contact cells, this is especially important for back contact cells). The ratio should typically be greater than five.
Because cell thickness cannot be reduced easily without compromising mechanical yield, for current back contact back junction solar cells the emphasis is to use a very high lifetime material. And while this may result in a larger diffusion length, using a high lifetime material also increases the substrate cost. However, by using thin cells, the diffusion length does not have to be as high, resulting in an ease in the material quality requirements and thus the cost of the cell. This cost reduction is in addition to the obvious cost reduction of using less silicon. Thus, a back contact/back junction cell on a very thin crystalline silicon substrate has both a large cost and performance advantage.